Remote control system



6. w. BAUGHMAN REMOTE CONTROL SYSTEM Filed April 24, 1937 Office Field Jzall'on.

96 51? Y wok Q LB T: 1

Grid Volts (0p relay Current.) v

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1a. Office Field 570((012 2 ma /50 aw 29 C Y1 Fly. 2.

INVENTOR George W 912mm HIS ATTORNEY Fatentecl July 4, 1939 UNITED STATES PATENT OFFICE REMOTE CONTROL SYSTEM Application April 24, 1937, Serial No. 138,794

Claims.

My invention relates generally to remote control systems, and more particularly to systems of the above character in which code signals are transmitted over a line which is relatively long 5 electrically, such, for example, as a long distance cable line having considerable distributed capacity as well as resistance. More specifically, the invention relates to the method, as well as apparatus, for decreasing distortion of the received signals, and, accordingly, for increasing the maximum length of the cable or other line over which code signals may be effectively transmitted and accurately reproduced.

My invention is an improvement on the invention disclosed in the copending application filed by Lloyd V. Lewis on April 17, 1937, Serial No. 137,500, for Remote control systems. As in the case of the aforementioned copending application, my invention is particularly adapted for,

though in no manner limited to, centralized traffic controlling systems for railroads employing a normally closed line circuit connecting a dispatchers office with a plurality of field stations located at spaced points along the line. The code signals employed for transmission from the oflice to the stations and from the stations back to the office may be of any suitable character, but in the illustrative embodiments which I have herein shown these signals are formed by'interrupting or varying the flow of current from a line battery which will usually be located at the dispatchers office.

It will be understood that the method and apparatus embodying my invention, though explained with reference to a railway trafilc controlling system for a clearer understanding of the invention, are of general utility in signal transmission systems and may be employed Wherever suitable current impulses must be ef- 44) fectively transmitted over an electrically long transmission line.

One object of my invention is to extend the effective length of the territory which may be controlled by code signals. Another object of 45 my invention is to provide reliable operation of the receiving relays, notwithstanding relatively large variations in the current or voltage level at which the line is being operated.

Other objects and features of my invention will become apparent as the description progresses.

I shall describe two forms of apparatus embodying my invention, and shall then point out the novel features thereof in claims.

In the accompanying drawing, Fig. 1 is a diag'rammatic view illustrating a simplified transmission line, embodying my invention, in which the transmitter is located at a remote station and the receiver is located at the ofiice.

Fig. 1a shows typical curves of grid voltage or current for the receiver of Fig. 1, which curves are used in explaining the principles underlying my invention.

Fig. 2 is a diagrammatic View showing a modified form of the apparatus of Fig. 1, also embodying my invention, in which two-way transmission may take place and in which the receiving relay at the field station is controlled in a manner similar to that of the receiving relay at the oflice.

Similar reference characters refer to similar parts in each of the several views.

The designations for the various relays disclosed herein are the same as for the corresponding relays shown in the copending application of Lloyd V. Lewis, Serial No. 137,500, hereinbefore referred to, as these relays are provided for the same purposeand perform similar functions to those described in said copending application. That is, the reference character R designates the receiving relay and is identified by the pre fix 0 when located at the control ofiice. The reference character T with a corresponding prefix similarly designates the corresponding transmitting relay for delivering'code impulses to the line circuit, while the reference character M desigtion of Lloyd V. Lewis, Serial No. 600,786, filed March 23, 1932, for Remote control systems.

In systems of this general character, the code signals are formed by opening and closing the line circuit at time spaced intervals by a transmitting relay, such as relay T. If the line is electically short, they variations in line current due to operation of the transmitting relay correspond in form to thevariations of the impressed voltage and, therefore,'are generally rectangular in form.

Reliable response "of receiving apparatus is, of

course, not difficult to obtain with current waves of this form. In a long cable line, however, having a relatively large amount of distributed capacity (shown concentrated at condenser c in Fig. l, for simplicity) the line current at a point remote from the point of transmission will depart materially from the rectangular form and may build up so slowly or die down so gradually, or both, as to materially distort the code signals and thereby decrease the certainty of operation of the receiving equipment.

In a system wherein a line battery located, for example, at the oflice end of the line is used for transmitting from both ends of the line, the condition which first limits the maximum distance over which satisfactory transmission may take place is the distortion at the control oflice of the codes formed by alternately opening and closing the line at the most remote field station of the system. This condition is exemplified by the apparatus of Fig. 1 in which the single line battery LB and receiving relay OR are located at the oflice and transmission takes place from a remote field station at the end of the line by alternately opening and closing the line circuit at that point.

In certain systems proposed heretofore in which the receiving relays are connected directly into the line circuit, these relays are responsive to the magnitude of the line current and, accord ingly, are susceptible to changes in the line current due to fluctuation in the line battery voltage, etc., and it follows, therefore, that successful operation of a long cable line under such conditions requires accurate control of the "line current.

When transmission of a code from a field station is initiated, the master relay M first picks up and disconnects the portion of the line circuit leading to more remote stations, while the portion of the line circuit leading to the oflice is maintained closed through a branch path which includes front contact l2 of relay M, back contact 2 of the'transmitter relay T, wire 3|, and a resistor M of substantially the same resistance as the portion of the line that has been disconnected. At the same time, relay M energizes the transmitting relay T by means of transmitting apparatus which forms no part of my'present invention and is therefore, not shown in the drawing. Energization of transmitter T opens the line circuit at back contact 2 to transmit an off impulse. The release of relay T, caused by operation of the transmitting apparatus hereinbefore referred to, results in the closing of back contact'2 of relay T, thereby closing the line circuit over front contact l2 of relay M, back contact 2 of relay T, and shunt wire 3| to form the on impulse of the code.

In a long cable line employing no corrective apparatus for shaping the line current, when contact 2 of transmitter T opens to transmit an off impulse to the oflice, an appreciable time will elapse before the effect of the opening of the contact becomes manifested at the office. This delay is due to the time consumed in charging the line through a considerable portion of the line resistance before the line current at the oflice will diminish to the release point of the office line relay.

The line current in the aforementioned line will diminish along a curve of the general character indicated by curve a in Fig. 1a. This curve is illustrative of the manner in which the line current (or the potential drop across a portion of the line) varies at the oflice end of a long cable line during transmission of an on impulse followed by an off impulse from a field station at the other end of the line. It will be apparent from an inspection of curve a that the increase in line current during the on impulse is more rapid than the decrease of the line current during the off impulse. Accordingly, as mentioned hereinbefore, the limiting condition as to the maximum distance of transmission is the distortion due to slowness in decay of the line current.

My invention provides means for minimizing the effect of this distortion by rendering the receiving relay responsive not merely to the magnitude of the line current, but to a combination of the magnitude as well as the rate of change of the line current. This result is accomplished by the apparatus of my invention by the production of a voltage which varies in accordance with the rate of change of the line current, and by the combination of this voltage with a second voltage which is proportionate to the instantaneous value of the line current, and by operating the receiving relay by a current which corresponds to the resultant voltage, so that both the rise and the decay of the current delivered to the relay are greatly accelerated.

For example, by adding to the voltage which corresponds to the line current a component which is a function of the rate of increase of this current, therise of the resultant current is made to take place very quickly. Similarly, by subtracting from the voltage corresponding to the line current a component which is a function of the rateof decrease of the current, the decay of the resultant current is likewise markedly accelerated.

One form of apparatus which may be used for producing the desired result is that shown located at the office in Fig. 1. This apparatus includes a line resistor 32 and a transformer TR having its primary winding 26 connected in series in the line circuit. Transformer TR and resistor 32 take the place of the usual receiving relay in the line circuit and conveniently, the total impedance of the combination may be made to match the impedance of the relay which is replaced thereby. With the connections as indicated in Fig. 1, the potential drop across resistor 32 will reflect the magnitude of the line current, whereas the potential induced in winding 21 will reflect the rate of change as well as the direction of the change of this current.

The windings of transformer TR are so ar ranged that upon anincrease in the line current, the voltage induced in winding 21 is in the same direction as the potential difference across resistor 32. It follows, therefore, that when a decrease in the line current takes place, the induced voltage will oppose the potential difference across this resistor.

The combined potential from across winding 21 and resistor 32 is impressed across the input circuit of a suitable vacuum tube VT, the output circuit of which includes a suitable source of plate current PB- as well as the office receiving relay OR. The apparatus is so proportioned that under normal conditions, when no station is transmitting and the line circuit is closed at the remote field station, the potential difference across resistor 32 is .made sufficient to properly maintain relay OR inthe plate circuit of the tube in its energized position. The relay OR may be of the usual neutral type, if desired, but more sensitive operation will be bad if this relay is made of the biased polar type,*as shown in the drawing.

It will now be apparent that when the line current starts to decrease, the drop across resistor 32 will decrease proportionately, but in adddition, the potential across the transformer secondary will oppose the drop across the resistor 32 so that the resultant voltage applied to the grid circuit of the tube will decrease very rapidly. The plate current of the tube will, therefore, decrease rapidly so as to promptly release the relay OR. Conversly, upon build-up of the line current, the induced secondary voltage will add to the increasing potential difference across resistor 32, thus providing quick pick-up of the receiving relay. These operations are illustrated in the curves of Fig. 1a which correspond in shape with oscillograph records made with apparatus set up in accordance with Fig. 1.

Referring to Fig. 1a, curve a represents the potential drop across resistor 32 alone, and thus reflects accurately the manner in which the line current varies at the ofiice end of the line when the remote field station is transmitting and the corrective apparatus is not employed. Curve b represents the potential which is induced in the output winding 21 of transformer TR as a result of variation of the line current along the curve a. It will be noticed that a sharp and relatively high positive peak occurs in curve b during the initial build-up of the line current. Moreover, a considerable negative peak quickly appears as soon as the line current begins to decay. Obviously, the energization of the grid circuit of the tube VT and, accordingly, the energization of the relay OR in the plate circuit thereof, is in accordance with the resultant, or sum, of the two curves a and b, that is, in accordance with curve 0.

Curve 0 reflects the general manner in which the current varies in the office receiving relay OR of the apparatus embodying my invention during the transmission of one on impulse followed by one off interval transmitted from the other end of the line. The advantage of curve 0 over curve a from the standpoint of quick and positive operation of the receiving relay will become immediately apparent from an inspection of curve 0 in which the current builds up quickly to a high value to insure pick-up during the on impulse, and drops rapidly to below zero to insure release during the off interval. Thus, it

is obvious that the relay OR will repeat with a high degree of accuracy the code being impressed on the line at a remote field station.

The particular level to which the current in curve 0 drops depends, of course, on the relative magnitude of the induced voltage as compared with the drop across resistor 32, as well as on the constants of the tube VT and the particular portion of the tube characteristic within which the operation takes place.

One feature of the centralized traflic control system of the Lewis application, Serial No. 600,786, hereinbefore referred to, is that if the control station and a field station start transmission at the same time, only the ofii'ce transmitter retains command of the line, so that two codes initiated simultaneously will be transmitted one at a time, inorder. The correct operation of this feature of the system requires that' the time lag in the release of the receving relay at one end of the line in response to the opening of the line by the transmitter at the opposite end of the lin be less than the initial short off period of a code, and it follows, therefore, that the time lag characteristic of a cable line imposes a limit upon the available transmission distance for this reason as well as because of signal distortion. This limitation, however, is largely removed by the apparatus of my invention, which has the effect of greatly reducing the apparent time constant of the line. This will be readily apparent by comparing the right-hand portion of curve a, Fig. 1a, which illustrates the decay in the line current when the line is opened at the distant end, with the corresponding portion. of curve c of the same view, which shows that the relay current decreases much more rapidly, just as if the time constant of the line had been greatly reduced. By proper design of the corrective apparatus, the rate-of-change curve I) can be shaped to provide almost instantaneous response of the receiving relay, thus largely eliminating this particular limitation on the maximum distance of transmission as well as improving the transmission over shorter distances.

In connection with Fig. 1a, a simple mathematical relation is found to exist between the current itself and its rate of change when a condenser is being charged through a resistor, which is approximately the condition existing when the field station in Fig. 1 opens the line at back contact 2 of relay T during transmission. In such a circuit, the rate of change of current is directly proportional to the magnitude of the current, the equation being d1 Cr 1 where i is the current, and r is the resistance through which the condenser of capacity C is being charged.

From this relation, it is apparent that once transformer TR and resistor 32 in Fig. 1 have been properly proportioned, proper balance between the induced voltage and the resistor drop will exist irrespective of the actual magnitude" of the line current. In other words, curve a, Fig. 1a, corresponding to i and curve I) corresponding to will have a constant ratio for a given length of line. In an actual installation, the apparatus would be so proportioned as to provide the proper balance when the most distant field station is transmitting. With nearer stations transmitting, the potential induced in winding Zl may predominate to a considerable extent over the potential drop in resistor 32, but the result will be to provide even quicker release and pick-up of the receiving relay.

Having provided the foregoing general description, I shall now summarize the operation of the apparatus of Fig. 1 during the transmission of one element of the code, transmission of other code elements being identical and readily understandable from the description which follows.

Assume that relay M has been energized and has caused the line circuit to be opened at back contact 2 by energizing the transmitting relay '1. The line current at the office will decrease according to the right-hand portion of curve a, Fig. la, and the secondary voltage of transformer TR will be according to the right-hand portion portion of curve 0.

of curve I) and will oppose the potential difference across resistor '32 so as to effect a quickrelease of the receiving relay OR, the relay current being in accordance with the right-hand When the line circuit is closed at back contact 2 at the field station, the line current at the office will increase according to the left-hand portion of curve a and the secondary voltage and resistor drop will be additive. The relay current will rise according to the lefthand portion of curve 0 so that relay OR will pick up quickly. As the line current a approaches the steady state, the drop across resistor 32 provides sufficient input voltage to tube VT to maintain the current through relay OR at its normal value.

Referring now to Fig. 2, the apparatus of this figure provides for transmission both from the remote field station to the ofiice and from the office to the field station. Relay OR is operated by the line signals when transmitting from the ofiice as well as when receiving codes from the field stations, but in the former case, since it adjoins the transmitter relay contact I which is delivering the code to the line, no distortion correcting device is required. On the contrary, it is preferable in this case to supply relay OR with current of a wave form substantially identical to that delivered to the line. To accomplish this, I have provided a contact28 on the office master relayOM, which contact short-circuits the output winding 21 of transformer TR whenever relay OM is energized, that is to say, whenever the oflice takes control of the line to transmit a code. In this case it will be apparent that the current through relay OR will be of the same form as the potential difference across resistor 32 and therefore of the same form as the current delivered to the line by transmitter relay OT.

If desired, contact 28 may be used to shortcircuit the primary winding 26, instead of the secondary in order to eliminate transformer TR from the line circuit.

During transmission from the remote field station in Fig. 2, relay OM is deenergized and contact 28 thereof is open, so that under this condi tion the ofiice receiving apparatus functions in the same manner as described in conjunction with Fig. 1.

The receiving equipment at the field-station in Fig. 2 comprises corrective apparatus .similar to that which is used at the office. That is, the apparatus provides for response of the receiving relay R in accordance with both the magnitude and the rate of change of the line current. Front contact 28 of relay M short-circuits winding 21 of transformer TR for the same purpose described hereinbefore, whenever transmission from the field station is taking place.

The field station transmittingequipment in Fig. 2 differs somewhat from that of Fig. 1.in that certain corrective apparatus is provided for charging the line capacity from the field station end of the line during the off code intervals, when transmitting from that end, of the form disclosed in the copending Lewis application, Serial No. 137,500, hereinbefore referred to. This corrective apparatus comprises an impulse transformer which is energized from a source of unidirectional current having the terminals 3-0, over contacts 8 and IU of relays M and T, re-

spectively. When front contact ID of relay T closes, the flux due to the input winding 29 is rapidly reversed and an impulse voltage from the output winding 30 is impressed across the line through the rectifier 6 and compensating resistor r to aid battery 'LB in charging the line capacity. In this manner, the line is charged more rapidly when relay T picks up, and a more symmetrical wave form of current is impressed on the line, thus further improving transmission from the field stations to the ofiice. Fig. 2 also includes a connection from line Y over front contact I of relay OT to line Z for short-circuiting the line through a resistor when relay OT picks up, as described in said Lewis application, for effecting a corresponding improvement in transmission from theofiice to the field stations.

The system shown in Fig. 2, therefore, provides corrective apparatus for shaping the wave of the line current both at the receiving as well as the transmitting point of the system, the apparatus provided at the transmitting point making it possible to obtain symmetrical transmission in either direction. The field station in Fig. 2 is shown as an intermediate station, and the line circuit is shown as extending beyond this location, as indicated by the line resistors r-r. Obviously, however, this station could just as well have been an end station, as in Fig. 1, it being understood that the corrective apparatus embodying my invention-may be applied with equal facility either to the office equipmentor the equipment at one or more, or all, of the field stations of an electrically long transmission line.

Although I have herein shown and described only two forms of remote control systems embodyingmy invention, it is understood that various changes and modifications .may be made therein within the scope of the appended claims Without departing from the spirit and scope of my invention.

Having thus described my invention, what I claim is:

1. In combination with anormally closed line circuit, having relatively high electrostatic capacity, a source of unidirectional current included in said line circuit at one end thereof, a transmitting relay at the other end of said line circuit for transmitting code current impulses from said source over the line circuit, a receiving relay at said one end of the line circuit responsive to the individual impulses of said code transmitted over .said line circuit and having an operating winding, means associated with said line circuit .and independent of said receiving relay for establishing a first potential difference proportionate to themagnitude of the line current, means. also associated withsaid line circuit for establishing a second potential difference proportionate to the rate of change of said line current, and means for combining said two potential differences and impressing them across said operat ing winding to thereby enable said receiving relay to respond more accurately to the individual code impulses transmitted over said line circuit.

2. In combination with a normally closed line circuit having relatively high electrostatic capacity, a source of unidirectional current for energizing said line circuit, a transmitter for transmitting code current impulses from said source over said line circuit, a receiving relay responsiveto the individual code impulses transmitted over said line circuit and having an operating winding, a resistor included in said line circuit, a transformer having its input winding also included in said line circuit, means for connecting the outputwinding of said transformer with said resistor in a direction such that the potential difference induced therein aids the potential difference across said resistor during those portions of said transmitted impulses when the current therein is increasing; and means for combining said two potential differences and impressing them across said operating winding to thereby enable said receiving relay to respond more accurately to the individual code impulses trans mitted over said line circuit.

3. In combination with a normally closed line circuit having relatively high electrostatic capacity, a source of unidirectional current for energizing said line circuit at one end thereof, means at the other end of said line circuit for opening and closing a shunt across the line circuit at code intervals to thereby transmit code impulses from said source over said line circuit, a receiving relay at said one end of the line circuit responsive to the individual impulses of said code and having an operating winding which is excluded from said line circuit, a resistor and a transformer both connected in series in said line circuit between said source and the outgoing line at said one end of the line circuit, and means for combining the potential difierence across said resistor with that across the output winding of said transformer and impressing them across said operating winding to thereby enable said receiving relay to respond more accurately to the individual code impulses transmitted over said line circuit.

4. In combination with a line circuit having relatively high electrostatic capacity, a source of unidirectional current for energizing said line circuit, a first transmitter at a first location and a second transmitter at a second location remote therefrom for transmitting code current impulses from said source over said line circuit, a receiving relay at said first location responsive to individual impulses of said code transmitted over said line circuit by said second transmitter and having an operating winding, means for establishing a first potential difierence proportionate to the magnitude of the line current, other means for establishing a second potential difference proportionate to the rate of change of said line current, means for combining said two potential difierences and impressing them across the operating winding of said receiving relay to enable said relay to respond more accurately to the individual impulses of said code transmitted by said second transmitter, and means effective during transmission by first transmitter for rendering said other means inoperative to thereby reduce distortion of the current in said receiving relay when its adjacent transmitter is operating.

5. In combination with a line circuit having relatively high electrostatic capacity, a source of unidirectional current for energizing said line circuit, a first transmitter at a first location and a second transmitter at a second location remote therefrom for transmitting code current impulses from said source over said line circuit, a receiving relay at said first location responsive to individual impulses of said code transmitted over said line circuit by said second transmitter and having an operating winding, a resistor at said first location for establishing a first potential difference proportionate to the magnitude of the line current, a transformer at said first location for establishing a second potential difference proportionate to the rate of change of said line current, means for combining said two potential difierences and impressing them across the operating winding of said receiving relay to enable said relay to respond more accurately to the individual impulses of said code transmitted by said second transmitter, and means effective during transmission by said first transmitter for shortcircuiting said transformer to eliminate the effect of said second potential difierence and thereby reduce distortion of the current in said receiivng relay when its adjacent transmitter is operating.

GEORGE W. BAUGHMAN. 

